N-methyl-D-aspartate (NMDA) receptors are ligand-gated cation-channels embedded in the cell membranes of neurons. Overactivation of NMDA receptors by glutamate, their natural ligand, can lead to calcium overload of cells. This triggers a cascade of intracellular events that alters the cell function and ultimately may lead to death of neurons [TINS, 10, 299-302 (1987)]. Antagonists of the NMDA receptors may be used for treating many disorders that are accompanied with excess release of glutamate, the main excitatory neurotransmitter in the central nervous system.
The knowledge on the NMDA receptor structure, function and pharmacology has expanded owing to recent achievements of the molecular biology. The NMDA receptors are heteromeric assemblies built up from at least one NR1 subunit and at least one of the four different NR2 subunits (NR2A-D). Both spatial distributions in the CNS and the pharmacological sensitivity of NMDA receptors built up from various NR2 subunits are different. Particularly interesting of these is the NR2B subunit due to its restricted distribution (highest densities in the forebrain and substantia gelatinosa of the spinal cord). Compounds selective for this subtype are available [Curr. Pharm. Des., 5, 381-404 (1999)] and have been proved to be effective in animal models of stroke [Stroke, 28, 2244-2251 (1997)], traumatic brain injury [Brain Res., 792, 291-298 (1998)], Parkinson's disease [Exp. Neurol., 163, 239-243 (2000)], neuropathic and inflammatory pain [Neuropharmacology, 38, 611-623 (1999)]. Moreover, NR2B subtype selective antagonists of NMDA receptors are expected to possess little or no untoward side effects that are typically caused by the non-selective antagonists of NMDA receptors, namely psychotomimetic effects such as dizziness, headache, hallucinations, dysphoria and disturbances of cognitive and motor function.
NR2B subtype selective NMDA antagonism can be achieved with compounds that specifically bind to, and act on, an allosteric modulatory site of the NR2B subunit containing receptors. This binding site can be characterized by displacement (binding) studies with specific radioligands, such as [125I]-ifenprodil [J.Neurochem., 61, 120-126 (1993)] or [3H]-Ro 25,6981 [J. Neurochem., 70, 2147-2155 (1998)]. Since ifenprodil was the first, though not sufficiently specific, known ligand of this receptor, it has also been termed ifenprodil binding site.
Close structure analogs of the carboxylic acid amide derivatives of formula (I) are known from the literature. The Florida Center for Heterocyclic Compounds [Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, Fla., 32611-7200] provides milligram quantities of three compounds of formula (I) for biological testing: N-(4-bromophenyl)-4-(phenylmethyl)-1-piperidineacetamide, 4-[[oxo[4-(phenylmethyl)-1-piperidinyl]acetyl]amino]benzoic acid and 4-[[oxo[4-(phenylmethyl)-1-piperidinyl]acetyl]amino]benzoic acid ethyl ester.
Oxo-ethylamino derivatives are described as intermediates for thrombin inhibitors [Bioorg. Med. Chem. Letters, 9, 925. (1999)]. The publication does not describe NMDA receptor antagonist effect.
N-(4-Benzoylphenyl)-4-(phenylmethyl)-1-piperidineacetamide is mentioned in U.S. Pat. No. 6,048,900 as selective neuropeptide Y receptor antagonist.
N-(2-Formyl-6-methylphenyl)-4-(phenylmethyl-1-piperidineacetamide is described in Australian patent application No. AU 639529 as an intermediate for carbostyril derivative which is useful as antiarrhythmics.
Aminoacetarylides are also known [Rev. Chim. (Bucharest), 33(7), 601. (1982); CA 97:174467a] as local anesthetic and antifibrillatory agents.
Piperidine derivatives and analogues substituted with phenols or phenol equivalents having NR2B selective NMDA antagonist activity are described in international patent application nos. WO 90/14087, WO 90/14088, WO 97/23202, WO 97/23214, WO 97/23215, WO 97/23216, WO 97/23458, WO 99/21539, WO 00/25109, European patent application No. EP 648744 A1 and in U.S. Pat. No. 5,436,255. Compounds containing 2-benzoxazolinone substructure with the same biological activity are described in international patent applications WO 98/18793 and WO 00/00197. Other NR2B selective NMDA antagonists having condensed heterocyclic structures are described in international patent application nos. WO 01/30330, WO 01/32171, WO 01/32174, WO 01/32177, WO 01/32179, 01/32615, WO 01/32634.
However, there continues to be a need for novel NMDA antagonists that target the NR2B receptor.